Christmas tree

ABSTRACT

A Christmas tree ( 16 ) is based on a dual bore sub-sea test tree. The Christmas tree consists of a Christmas tree housing ( 56 ), which connects to a wellhead ( 44 ), a Christmas tree valve block assembly ( 62 ) disposed in the housing ( 56 ) and a tree cap ( 84 ) which connects to the housing ( 56 ) and valve block assembly ( 62 ). In a preferred embodiment the valve block assembly ( 62 ) has a main production bore ( 70 ) with two valves ( 72,74 ) in series and two auxiliary bores ( 76,96 ). One auxiliary bore ( 96 ) has a valve ( 98 ) for facilitating control of the annulus bore ( 96 ) whereas the other annulus bore ( 76 ) has no valve and provides a pathway for an electrical submersible cable ( 66 ) to a pump. The valves ( 72,74,98 ) are actuatable via control from an umbilical ( 94 ) and provide the facility to seal the production and annulus access bores to meet statutory requirements. A tubing hanger is not required.

The present invention relates to a xmas tree structure for use inextended well tests. In particular, the invention relates to asimplified horizontal xmas tree structure for use in such tests and to amethod of installing such a tree for use in such extended well tests.

Extended well tests (EWTs) are procedures which can last up to 180 daysand are carried out in order to assess the viability of a producingreservoir and to obtain accurate reservoir data over a prolonged period.Extended well tests are common for on-shore locations requiring littlemore than a conventional well test package. Off-shore packages aresomewhat different due to tighter regulatory, product and effluentspecifications. Due to the extended flow periods, crude oil is normallyrecovered for commercial sale from the extended well test. Off-shoresystems are highly sensitive to well performance because the cost of arig and export system has to be accounted for. It is of considerableimportance to be able to minimise the cost of conducting extended welltests and the apparatus used in the tests without comprising theaccuracy of reservoir data or compromising safety.

Extended well tests are typically conducted using a traditional sub-seatest tree and BOP stack. However, if additional wells are also to betested, or if an injectivity test is to be conducted, it will benecessary to deploy a sub-sea xmas tree to provide well pressurecontrol. For applications in which well testing involves testing morethan one well, the term “extended appraisal test” (EAT) is used in placeof the term extended well test (EWT). Traditional xmas tree arerelatively expensive and require additional accessories and tools suchas tubing hangers and tubing hanger running tools and a workover systemin order to conduct the extended well test further adding to the rigtime and overall cost of conducting the well test.

An object of the present invention is to provide a pressure controlsystem embodied in a simplified structure and a method of its deploymentwhich obviates or mitigates at least one of the disadvantages associatedwith existing xmas trees.

This is achieved by providing a simplified xmas tree based on a dualbore sub-sea test tree. A separate tubing hanger is not required and themain bore of the sub-sea test tree is used as the production bore. In apreferred embodiment, an axial bore, which includes an isolation valve,is provided for communication with the well annulus and an additionalaxial bore is occupied by an electrical power cable for supplying anelectrical submersible pump (ESP).

According to a first aspect of the present invention there is provided axmas tree for use in a variety of well applications, said xmas treecomprising:

a generally cylindrical xmas tree housing defining a generallycylindrical bore, a separate xmas tree valve disposed within thecylindrical bore of said xmas tree housing and engaged with the xmastree housing, and a xmas tree cap for engaging with said valve block, orsaid xmas tree housing, the xmas tree cap being coupled to a rig viavarious flowline and umbilical services to facilitate control of thexmas tree in conveying fluids between surface mounted process facilityand a downhole reservoir,

the xmas tree valve block having a main production bore and at least oneauxiliary bore, the main production bore having at least two valvesaxially spaced therein, said valves being operable to seal the bore andprovide well barriers, said at least one auxiliary bore providing anannulus access bore and containing a single valve operable to seal theaccess bore and provide a well barrier in the annulus.

Conveniently, the xmas tree valve block has a main production bore andtwo auxiliary bores. One auxiliary bore provides the annulus access boreand has a valve therein and the other auxiliary bore has no valve but isused for receiving a power cable for driving an electrical submersiblepump.

In the absence of a requirement for an electrical submersible pump, nopower cable is used and a single axial bore for the cable is providedor, if two bores are present, the additional bore is blocked and sealed.A single axial annulus access bore is provided for providing annuluscontrol regardless whether or not there is a power cable disposed insaid auxiliary bore.

It will also be appreciated that each of the bores has sealing means toseal the bores so that leakage of fluid from the bore is prevented whenthe tree cap is made up to the valve block and/or valve housing.

Preferably also, said valves in series within said production bore andthe annulus bore are ball valves. Alternatively, said valves are flappervalves, disc valves or plug valves.

Preferably also, the housing engages with a standard wellhead connectorto enable it to be attached to a sub-sea wellhead and the valve block isproportioned and dimensioned to fit in within the bore of this housingand the housing is adapted to be received by the tree cap so thatconnection is made between parts of the production bore, and parts ofthe two auxiliary bores to provide effective connection for these bores.The connections in the production and annulus bore are hollow pin andsocket connections. The connections in the bores for electricalcommunication are of a wet mateable connection in the power and signalcables. The connections in the controls parts are also by hollow pinsand sockets.

Conveniently also, locking means are provided between the xmas treehousing and the valve block assembly which are actuated when the treecap fits on to the valve block to lock the valve block to the xmas treehousing, and to provide fine alignment between the tree cap and thehousing. Conveniently, the locking means are radially acting dogs whichare actuated by axial movement of a ring within the tree cap lockingmeans. The top surface of the upper end of the valve block, which islevel with the top of the housing, is provided with profiles toaccommodate:

1) the sealing of the production and annulus bores,

2) the sealing of the hydraulic ports required for the control of theactuation of xmas tree and other well functions,

3) repeatable make and break of connections in the communications ofelectric signals between the surface and the electrical equipmentlocated within the well,

4) repeatable make and break of connections in the communications ofelectrical power between the surface and electric submersible pump.

The external surface of the upper end of the valve block is providedwith at least one shaped keyway to ensure correct angular alignmentbetween the xmas tree and the tree cap to facilitate correct engagementof the various connections at the top surface of the tree cap asdescribed above.

Conveniently also, the xmas tree housing may be coupled to a guide framewhich is adapted to guide and receive the tree cap when installed sothat there is effective and efficient coupling between the tree cap andthe tree housing and valve block. The guide base is fitted to thewellhead system, and a protective frame is attached to the tree housingwhich takes the guidance from the wellhead, into the tree, and henceinto the tree cap.

A permanent guide base, attached to the wellhead system, including aside guide frame with guidepost spacing of the same dimensions as theguide frame coupled to the wellhead so that when the tree cap is removedfor workover mode, the tree cap can be moved to one side and parkedadjacent to the wellhead.

According to another aspect of the present invention there is provided axmas tree structure comprising:

a xmas tree housing coupled to a sub-sea wellhead;

a xmas tree valve block located in said xmas tree housing, said xmastree valve block having a production bore with at least two valvesdisposed in series and at least one auxiliary bore for facilitatingannulus access control, said at least one auxiliary bore having a valvetherein, a xmas tree cap coupled to the xmas tree valve block and tosaid xmas tree housing, locking means disposed in said xmas tree tofasten the xmas tree housing, xmas tree valve block and said xmas treecap together, said respective xmas tree valves being remotely operableto be selectively moved between open and closed position to control flowor liquid through said xmas tree structure.

Conveniently, there are two auxiliary bores; one annulus access bore anda further auxiliary bore with at least one valve therein and a furtherauxiliary bore for receiving a power cable for supplying power to adownhole electrical submersible pump (ESP).

According to a further aspect of the present invention, there isprovided a method of installing the xmas tree on a wellhead for use in avariety of well tests, the tree housing having a blow-out preventermeans initially coupled thereto in place of a tree cap, said methodcomprising the steps of:

a) providing a xmas tree housing coupled to the wellhead, the xmas treehousing having a top part adapted to be coupled to the blow-outpreventer connector,

b) running a xmas tree valve assembly through said BOP stack forengaging with the xmas tree housing and being operable for providing awell barrier to the flow of well fluid,

c) closing the well barrier to provide well isolation once the xmas treevalve assembly is located in said xmas tree housing,

d) removing the blow-out preventer stack from the xmas tree housing,

e) running in a xmas tree cap and securing the xmas tree cap to the xmastree housing and xmas tree valve assembly to provide coupling from theproduction and annulus bores to the surface and from auxiliary ports andbore to the surface, the arrangement being such that the valves in theproduction and annulus bores can be controlled to provide control ofwell fluid and xmas tree functionality.

Preferably also, the method includes the step of using the xmas tree ina workover mode, said method including the additional steps of removingthe xmas tree cap and parking the xmas tree cap on a further guide frameadjacent to the wellhead and xmas tree housing, and coupling a blow-outpreventer stack on top of said xmas tree housing with said xmas treeassembly contained therein.

These and other aspects of the invention will be better understood fromthe following description when taken in combination with theaccompanying drawings in which:

FIG. 1 depicts a diagrammatic representation of a semi-submersible rigcoupled to two EAT trees (extended appraisal test tree) simplified xmastrees in accordance with an embodiment of the present invention shownmounted on wellheads;

FIG. 2 is a sectional elevational view through part of a simplifiedhorizontal xmas tree housing in accordance with an embodiment of thepresent invention installed on a wellhead with a BOP stack shown coupledto the xmas tree;

FIG. 3 is a view similar to FIG. 2 but depicts an insert tree valveblock being installed through the BOP stack into the xmas tree housing;

FIG. 4 shows the insert tree valve block of FIG. 3 landed within thexmas tree housing;

FIG. 5 shows a view similar to FIG. 4 with the BOP stack removed and asimplified xmas tree in accordance with the embodiment of the presentinvention being completed by the installation of a xmas tree cap;

FIGS. 6a and 6 b are respective perspective views of a lockdown assemblyand part of a valve block respectively for locking down the valve blockand providing fine angular alignment from the block to the xmas treecap;

FIG. 6c is an enlarged part section through the lockdown assemblylocated within the xmas tree housing;

FIG. 7 is a view similar to FIG. 5, but drawn to a larger scale, inwhich the xmas tree cap is shown coupled to the xmas tree housing todefine a complete simplified xmas tree ready for in production mode withthe flow line controls and umbilical attached to the xmas tree cap;

FIG. 8 is a sectional view taken on the line 8—8 through the structuralhousing of FIG. 7;

FIG. 9 is a detail of part of the dual bore tree shown in FIG. 7 andtaken on the lines 9—9 of FIG. 8 to demonstrate the second auxiliarybore, providing communication with the annulus and showing theproduction and annulus bores with isolation valves, and

FIG. 10 is a view of the extended well test xmas tree in workover modewhere the xmas tree cap has been removed and parked adjacent to thepermanent guide base (PGB) and a BOP stack is connected to thestructural housing.

Reference is first made to FIG. 1 of the drawings which is adiagrammatic illustration of an extended appraisal test (EAT) tree froma semi-submersible rig 10 in which there is simultaneous production fromtwo separate wells 12,14.

Wells 12 and 14 have associated extended appraisal test (EAT) treesimplified horizontal xmas trees 16,18 which are located on respectivewellheads 20 and 22. A horizontal tree is one in which the completioncan be pulled out of the well without the need to recover the tree. Eachxmas tree 12,14 is coupled to the semi-submersible 10 via various lines,generally indicated by reference numeral 24, a flexible production riser26, an electrical power cable 28 and a controls umbilical 30. Theumbilicals are connected to a respective rig-mounted process system 34,control system 36 and electrical power distribution system 38 viarig-mounted EQDC (Emergency Quick Disconnect/Connect) units. Liquidswhich are produced during the extended well test passes through theprocess system, and oil is pumped to a tanker 41 via EQDC 39 add exportline 40.

The simplified xmas trees 16,18 on the wellheads, which will be laterdescribed in detail, are based on a dual bore sub-sea test tree andallow an extended appraisal test (EAT) to be conducted without theexpense and complexity associated with traditional xmas trees.

Reference is now made to FIG. 2 of the drawings which is an enlargedscale sectional elevation through the wellhead and the EAT xmas tree 16which consists of a xmas tree housing installed on a wellhead with ablow-out preventer BOP attached to a structural housing. Forconvenience, only the simplified xmas tree 16 will be described indetail although it will be understood that xmas tree 18 has the samestructure and operates in the same way. Mounted on the conductor housing20 of the wellhead system is a four post permanent guide base (PGB) 42which surrounds 18¾″ high pressure wellhead 44. A guide frame 46 fittedwith four funnels to capture the PGB guide posts 42 a is coupled ontothe xmas tree housing connector 48 which has dogs 50 which engage withan exterior profile 52 on the wellhead 44 to securely fasten the guideframe 46 and tree connector 48 to the wellhead 44. The wellheadconnector 48 has a connection point 55 for receiving a structural xmastree housing 56 as shown in FIG. 2. An internal cam profile (not shown)is provided at a pre-determined angular orientation with respect to theguide frame 42, to engage a key on the valve block to control theangular position of the valve block in relation to the PGB 42, in orderto achieve correct landing of the tree cap. A BOP stack 60 also having aguide frame 57, with funnels 58 which receive the posts 46 a of frame46, is shown mounted on the xmas tree housing 56. The arrangement ofFIGS. 2, 3 and 4 depict the installation or workover mode as compared toextended well test mode.

As mentioned above, a conventional type of horizontal tree wellhead isexpensive and it also requires that the tubing hanger is run prior touse of the wellhead.

This is overcome using the arrangement shown in FIG. 3 in which aninsert xmas tree valve block, generally indicated by reference numeral62 is installed through the BOP 60 into the xmas tree housing 56. Inthis case it will be understood that the insert tree valve block 62 isbased on a dual bore sub-sea test tree (similar to the type disclosed inapplicant's co-pending International Published Application No.PCT/GB96/01115) which carries at its lower end tubing 64 and anelectrical cable 66 which is coupled to an electrical submersible pump(ESP), not shown in the interest of clarity. The installation procedurefor the insert xmas tree valve block 62 will be later described but itis sufficient to note that at present, with reference to FIG. 3, thevalve block 62, including the tubing 64 and the cable 66 which it iscarrying, passes through the BOP stack 60 and the xmas tree housing 56.

Reference is now made to FIG. 4 of the drawings which is similar to FIG.3 except that the insert tree valve block 62 has been landed on anannular landing shoulder 68 within the structural housing 56 whereby itadopts the position shown in FIG. 4; that is, part of the xmas treevalve block 62 extends upwardly from the xmas tree housing 56 inside theBOP stack 60. It will be seen that the main bore 70 of the valve block62 contains two 5″ ball valves 72,74 in series similar to the sub-seatest tree and, as shown in FIGS. 8 and 9, a smaller single valve 98 inthe annulus bore 96, and an auxiliary 2″ annulus bore 76 provides apathway for the electrical submersible cable 66. The ball valves 72,74are qualified to a relevant xmas tree design standard, for example AP117D. It will be understood that the xmas tree ball valves areuni-directional sealing valves which may be used in a bi-directionalapplication if required. Each ball valve 72,74 is “failsafe closed”fitted with a spring package 75,77 respectively to return the valve tothe closed position on loss of control pressure. This is designed togive a valve closure time of approximately 30 seconds. Each ball valvehas a double acting hydraulic actuator (not shown in the interest ofclarity) whose axis is coincident with the axis of flow through thevalve. This means that pressurised hydraulic control fluid can beapplied to either the open or the closed side of the actuator. For auni-directional sealing application, a single control line conveyspressurised control fluid to the open side of the actuator. This opensthe valve against spring force, which spring force returns the valves tothe closed position, on cessation of the application of pressuremaintaining the valve in the open position. It will also be appreciatedfrom FIG. 4 that the xmas tree valve block 62 is run on a casing riser78 and the electrical cable connection is made via a power cable 80. Anumbilical (not shown in the interest of clarity) contains a bundle oftubular conduits for the conveyance of control fluids, and forcommunication with the well annulus. The xmas tree housing 56 has theannular landing shoulder 68 for receiving the xmas tree valve block 62as shown and when the valve block 62 is in the position as shown in FIG.4, it is then in the correct position. Once the xmas tree valve block isinstalled, the ball valves 72,74 and 98 can be actuated to a closedposition to provide well control barriers in the main bore 70, and theannulus bore 96 also as shown in FIG. 4, and the BOP stack 60 can thenbe removed.

Reference is now made to FIGS. 5 and 6a, b and c of the drawings; FIG. 5depicts installation of the xmas tree cap, generally indicated byreference numeral 84, after the BOP stack 60 has been removed, and FIGS.6a, 6 b depict the separate parts of a lockdown assembly by which thetree cap 84 is accurately aligned orientationally to the valve block 62and xmas tree housing 56 and FIG. 6c depicts the lockdown assemblylocated within the xmas tree housing 56.

The EAT tree cap connector 83 is mounted in a tree cap guide frame 85which has four guide frame funnels 86 (only two of which are shown) sothat when it is run, as shown in FIG. 5, the funnels 86 mate with theguide posts 46 a of guide frame 46, thereby providing coarse alignmentbetween the tree cap connector 83 and the xmas tree housing 56 and treevalve block 62. It will be seen that the tree cap connector 83 is acentral perforated cylindrical block within a conventional externallyattached wellhead connector, and which is of a similar structure to thewellhead connector 48, and locks to the structural housing 56 in thesame way as the wellhead connector 48 connects to the wellhead 44. Oncethe valve block 62 has been landed within the housing 56 a lockdownassembly 161, shown in FIG. 6a, is installed. This has an expandingprofile which engages a suitable internal profile 162 in the housingand, when set, locks the valve block 62 in the housing 56 in order toprevent its upwards movement due to well pressure acting on closedvalves 72,74 within the valve block 62. The lockdown assembly 161 has anouter body 161 a and a rotatable and axially movable, via engagement ofthreads 166 a,b (FIG. 6b), inner ring 169 for locking the assembly tothe housing 56, as will be later described, outer body 161 a has lowerkeys 163 (one of which is shown) which engage shaped keyways 165 on theexternal surface 167 at the upper end of the valve block, and similarupper keys 167 engage corresponding keyways 159 in the tree cap 84, toprovide fine alignment to ensure the engagement of the hollow pins andsockets, in the connections in the halves of the production and annulusbores, the power and the signal electrical connectors and the controlsconnections, between the tree cap 84 and the insert valve block 62 aswill be described below. The assembly is run in and landed with thelower protruding keys 163 engaged with the keyway 165 in the valve block62. As best seen in FIG. 6c, rotation of the inner body 169 expands dogs171 to lock into a housing groove 162. With the dogs 171 engaged,further downward travel of the inner body 169 contacts the valve block62 to rigidise it in place within the tree housing 56. As shown in FIG.5, the tree cap connector 83 has parts which mate with the valve blockincluding the upper part of wet mateable connectors, generally shown bynumeral 88, for making the connections in the electric signals and powerto downhole equipment, generally shown by numeral 89, and hollow, pins,generally indicated by numeral 90, for connection with the sockets inthe production, annulus and controls ports bores generally shown bynumeral 92. It will be seen that the top 90 of the tree cap 84 containsa termination 93 for the flow line and controls umbilical. It will alsobe seen from FIG. 5 that the permanent guide base 42 has a sideextension, generally indicated by reference numeral 99, of the sameconfiguration as the guide base 42 for receiving the xmas tree cap 84from the xmas tree housing 56 in order to accommodate a workoveroperation which requires a BOP stack to be installed on the treehousing.

Reference is now made to FIGS. 7 and 9 of the accompanying drawingswhich depicts the assembled simplified xmas tree 16 based on the xmastree valve block 62 for providing control of reservoir fluids. The treecap 84 is mounted on the xmas tree housing 56 with dogs 91 engaging theexterior profile 93 in the same way as the BOP stack 60 was connected.When xmas tree is assembled there is connection between the parts of themain production bore 70, the annulus bore 96 and auxiliary bore 76. Inthis arrangement, which is the production mode, it can be seen that theflow line and umbilicals, generally indicated by reference numeral 94,are attached to the tree cap 84. It will also be seen in FIGS. 7 and 9that the 5″ ball valves 72,74, and 1½″ valve 98 are shown in the closedposition. However, it will also be appreciated that they may be actuatedvia the umbilical 94 from surface to open positions to permit reservoirfluid to flow through the main production bore 70, and to monitorpressure in the annulus bore 96. The electrical power cable 66, whichpasses through the auxiliary bore 76, allows power to be supplied fromthe surface to the downhole electrical submersible pump.

FIG. 8 depicts a section through the xmas tree housing 56 on line 8—8 ofFIG. 7 and it will be seen that there are three principal axial bores inthe insert valve block (control axial bores have been omitted forclarity); the main 5″ production bore 70 which has the two ball valves72,74 in series, the 1½″ annulus access bore 96, and the 2″ auxiliarybore 76, which receives the electrical power cable 66 coupled to thedownhole electrical submersible pump.

Reference is also made to FIG. 9 of the drawings which is a sectionalview taken along the lines 9—9 of FIG. 8 and shows a partial sectionalelevation of the tree showing through a detail 98 (shown in brokenoutline) of the xmas tree 16 of FIG. 7, depicting the production andannulus access bores 70,96 each with an isolation valve 74,98 (with thetubing omitted for clarity). It will be appreciated that with thisstructure, control of the annulus line 96 will be carried out in thesame way as using the dual bore sub-sea test tree with the resultingadvantages also being present in this arrangement.

Reference is now made to FIG. 10 of the drawings which shows the xmastree 16 in workover mode with the tree cap 84 removed and the flow lineand umbilicals part 94 parked on the extended guide base 99 disposedadjacent to the permanent guide base 42 and with the BOP stack 60 run toagain mate with the xmas tree housing 56 in the same way as shown inFIG. 4 of the drawings.

It will be appreciated that the xmas tree valve block 62 is run on theriser 78 as if it were a sub-sea test tree and landed on the annularlanding seat 68 within the xmas tree housing 56. It will also beappreciated that because the xmas tree valve block 62 interfaces withits deployment tool 121, and the xmas tree cap 84 having multiple axialpenetrations, it must be installed in a known orientation to thepermanent guide base guide posts 46 in order to ensure correctengagement with the tree cap 84 and the electrical power connector andthe usual seal subs. The orientation alignment between the tree 16 andthe tree cap 84 is achieved in step-like fashion with each successivestep controlling more closely the alignment.

As described above, rough alignment is established between the treeguide frame funnels 58 and the post on the PGB guide frame posts 46 a,and immediate alignment is established with the installation of the xmastree valve insert block 62 into the housing 56. Fine alignment is set upwith the installation of the lock-down assembly (161) in preparation forthe alignment required for the installation of the tree cap 84.

The insert valve block 62 is provided with means of achieving correctalignment. As described above with reference to FIGS. 6a and 6 b, thekeyways 165 on the external diameter of block 62 facilitates engagementwith keys 167 and orientational alignment of the latch/running tool.Similarly, at the bottom end, an orientational alignment key (not shown)is fitted to the xmas tree insert valve block 62 which interacts with aninternal bi-directional cam profile (not shown) within the lower end ofthe cylindrical xmas tree housing 56. The angular relationship betweenthe orientational aids or main set valve block 62, the keys at the topand the orientation key at the bottom are controlled.

The lock-down mechanism, or assembly 161 is run in to rigidise the valveblock 62 in place and also provides a fine orientational alignment. Asdescribed above, with reference to FIGS. 6a to 6 c, the underside of thelockdown mechanism 161 carries the same key profile 163 as the valveblock 62, whereas an upward facing key 167 at the top of the lock-downmechanism 161 provides a precision location for a matching keyway withinthe tree cap 84.

The aforementioned structure and overview of installation and operationwill give the reader an understanding of the structure and theinstallation procedure. However, for a better and more completeunderstanding regarding the running of the EAT xmas tree 16, it will beunderstood that prior to and during the running of the completion, thetree components are tested for function, orientation and pressureintegrity using a tree stump/shipping skip. It is anticipated thatdrilling is carried out conventionally with the BOP stack 60 locateddirectly on the wellhead 44 which may be of a conventional type, forexample Vetco SG-5 H-4.

A brief summary of the sequence of events regarding the running of theEAT simplified xmas trees 16,18 is as follows:

a) At the end of the drilling phase, the well is suspendedconventionally with the appropriate plugs/suspension string. The BOP 60is pulled and a wellhead corrosion cap (not shown) installed and the PGBguidelines are disconnected and retrieved.

b) In the event that a rig and completion equipment are available forinstallation of the completion immediately after drilling, it will beunderstood that the trip to run the corrosion cap may be eliminated.

c) The xmas tree housing 56 is then run using the running tool availablefrom the drilling phase and is locked on the wellhead 44 using either aworkover (W/O) umbilical attached to the hydraulically operated wellheadconnector by using a ROV “hot stab” connection system stabbed into asuitable receptacle on the guide frame of the xmas tree spool housing56. The workover umbilical may then be disconnected using the ROV.

d) The BOP stack 60 is then run in the conventional way on marine riser.

e) Once the BOP stack 60 is run and is in position, as shown in FIG. 2,the running tool is used to run in the hole and test the VX seal, therunning tool being available from the drilling phase and being fittedwith a cup tester which seals the top of the production casing or thecasing hanger in the wellhead.

f) Once the VX seal has been tested, the running tool and cup tester ispulled from the hole.

g) On surface the electrical submersible pump (ESP) is made up to thecompletion tubing and packer, the downhole chemical controls line ismade up and the ESP is then run in the hole on 7″ tubing/casing with theelectrical power cable and controls line being unreeled and strapped tothe production tubing as necessary until the correct length ofproduction tubing has been run in the hole.

h) The xmas tree valve block 62 will already have been made up to itsrunning tool 121, FIG. 10, on the surface and will have been stored(racked) back in the derrick. This valve block 62, the running tool 121and the umbilical 80 are now made up to the string 78. The downholeelectrical cable 66 cut and terminated at the termination point 130,131,FIG. 3, to tails from the wet mate connector halves 88,89 at the treecap 84 and the xmas tree valve block 62 and tested for continuity andfunction.

i) Assuming that the continuity and function is acceptable, the xmastree valve block 62 is run in the hole and production tubing and landedin the xmas tree housing 56 as shown in FIG. 4. In this position the ESPcan be tested as required.

j) The xmas tree valve block running tool 121, the running string andumbilical 80 are then retrieved from the hole.

k) The lockdown assembly 161 and running tool not shown are picked upand run in the hole on tubing or drill pipe and the valve block 62 isthen rigidised within the xmas tree housing 56 by turns of right handrotation of the lockdown assembly running tool and thereafter thelockdown assembly running tool is retrieved from the hole.

As described above, the tree cap 84 is run with the flow line, theumbilical and the electrical power cable attached to the tree cap, whichall need to be pulled out from storage drums located on the rig deck oron an adjacent vessel. Once these “flexible members” have been unreeledfrom the respective storage drums, the free ends are delivered andconnected to the EQDC system. It is necessary to have installed “keelhaul” lines from the storage reel locations into the moonpool area inorder to deliver the ends of the flexible members to the moonpool forattachment to the xmas tree cap 84.

The xmas tree cap 84 is positioned on the spider beams (not shown in theinterests of clarity) in the moonpool of the vessel and the respectiveconnections of the flow line, umbilical and electrical power cable 94are made up to the xmas tree cap 84 with the required bend restrictors,strapping and buoyancy modules being installed as required.

Thereafter, the connection is made up to the tree cap running tool 104,the spider beams are spread and the xmas tree cap 84 picked up and runin to land it on the xmas tree housing 56 as shown in FIG. 7 with theflexible members being strapped together at the various locations alongtheir length as necessary.

Once in the position shown in FIG. 7, the tree cap connector 84 islocked in place using alternative procedures described above for runningthe structural housing 56 and the system is tested. The tree cap runningtool 104 is then retrieved.

It will be appreciated that the tree may be retrieved using steps whichare basically the reversal of the running in steps.

The workover procedures are similar to the retrieval procedures exceptthat the tree cap 84 is not recovered to the surface provided that it isstill functional. The tree cap running tool 121 is run, the tree capconnector is released and the tree cap 84 lifted off the tree 62. A ROVis mobilised to disconnect the guidelines which are fitted with ROVreleasable post tops of a proprietary manufacturer (such as Regan GL4).

As best seen in FIG. 10, the tree cap 84 is skidded to one side, similarto running the BOP stack 60, and the guidelines are reconnected to asecond set of posts of guide frame 92 adjacent to the normal side posts46, thereby forming a “parking bay” adjacent to the well attached to theexisting permanent guide base 46. The tree cap 84 is set down over theposts of the parking bay and the tree cap running tool 104 disconnectedand retrieved to the surface. The ROV is remobilised to effectreconnection of the guidelines back to the well guide post tops.

This allows the BOP stack 60 to be re-run to land on the structuralhousing 56 of the tree, as shown in FIG. 10, and the completion and ESPcan be pulled as a normal retrieval operation. It will also beappreciated that the tree cap can be replaced as a reversal of theforegoing steps.

Various modifications may be made to the simplified xmas treehereinbefore described without departing from the scope of theinvention. For example, the preferred embodiment describes a xmas treewith an auxiliary channel which receives a cable for use with anelectrical submersible pump. However, it will be understood that thesystem could equally well be used in a well with natural drive in whichcase a conventional sub-sea test tree may be used as a valve block withthe annulus line performing its normal annulus path function. It willalso be understood that the ball valves may be replaced by any othertype of suitable valves, such as flapper valves, which are sufficient toprovide a sealing function in the production bore and the annulus bore.It will be understood that a single valve may be used in the xmas treebore but, for safety reasons, two valves in series are preferred in theproduction bore, although a single valve in the annulus bore isadequate.

It will also be understood that various diameters of xmas tree housingsand xmas tree valve blocks may be used. However, it will be understoodthat the xmas housing of 18¾″ nominal bore diameter which is the same asthat of the wellhead is particularly convenient and is designed toreceive a valve block assembly and machined in accordance with thediameter of a typical 5″×2″ sub-sea test tree. It will be appreciatedthat the dimensions of the bore and length of the tools may be varied inaccordance with specific requirements and that the dimensionshereinbefore described are exemplary only.

The connection between the xmas tree cap and the xmas tree housing maybe other than using a wellhead type of connector as hereinbeforedescribed. Any other suitable connector may be used which gives anappropriate sealing function which is sufficient to comply with thesafety requirements and it will be understood that the tree capconnector and structural housing connection which simulates the wellheadand wellhead connector arrangement is particularly convenient as theseproducts already exist aid have been well tested.

In the event of a requirement to test the tree valves from above, asecond control line is required to convey pressurised control fluid tothe closed side of the actuator. This creates an upward force whichcounteracts the hydrostatic pressure acting downward on the ball fromabove, and maintains contact between the ball and the seat toprevent-leakage, thus facilitating the test of the valve from above, ifso required.

It will also be understood that although the preferred used of thesimplified xmas tree structure is an extended well test, it willnevertheless be understood that the xmas tree may be used in an earlyproduction facility or even a permanent production facility and also forwater and gas injection operations where relatively quick andstraightforward access to a well is required without requiring theconsiderable expense of a traditional xmas tree.

Advantages of the simplified xmas tree in accordance with the presentinvention are that the structure can be readily and quickly installedand is capable of being used in a variety of well applications, such asextended well tests, extended appraisal tests and early productionfacility and gas and water injection. The xmas tree uses many existingwell components thereby minimising the cost and also utilises a dualbore sub-sea test tree to provide dual valve protection in theproduction bore, whereas the annulus bore is used to provide access intothe well annulus and an auxiliary bore is used to provide a facility forreceiving an electrical power cable for driving a electrical submersiblepump. In the situation where a power cable is provided through, anadditional bore is necessary to allow the installation of the electricpower cable for an ESP. This enables the simplified xmas tree to provideall of the functions of a traditional xmas tree. In addition, the xmastree can be readily assembled or the tree cap removed from the xmas treeto receive a BOP stack for use in the workover mode. A further advantageis that the dual valve xmas tree insert provides well barriers inaccordance with statutory offshore requirements.

What is claimed is:
 1. A xmas tree for use in a variety of wellapplications, said xmas tree comprising: a generally cylindrical xmastree housing defining a generally cylindrical bore, a separate xmas treevalve block disposed within the cylindrical bore of said xmas treehousing and engaged with the xmas tree housing, and a xmas tree cap forengaging with one of said valve block, and said xmas tree housing, thexmas tree cap being coupled to a rig via various flowline and umbilicalservices to facilitate control of the xmas tree in conveying fluidsbetween surface mounted process facility and a downhole reservoir, thexmas tree valve block having a main production bore and at least oneauxiliary bore, the main production bore having at least two valvesaxially spaced therein, said valves being operable to seal the mainproduction bore and provide well barriers, said at least one auxiliarybore providing an annulus access bore and containing a single valveoperable to seal the access bore and provide a well barrier in theannulus access bore.
 2. A xmas tree as claimed in claim 1 wherein thexmas tree valve block has a main production bore and two auxiliarybores.
 3. A xmas tree as claimed in claim 2 wherein one auxiliary boreprovides the annulus access bore and has a valve therein and the otherauxiliary bore has no valve but is used for receiving a power cable fordriving an electrical submersible pump.
 4. A xmas tree as claimed inclaim 3 wherein the housing engages with a standard wellhead connectorto enable it to be attached to a sub-sea wellhead and the valve block isproportioned and dimensioned to fit in within the bore of the housingand the housing is adapted to be received by the tree cap so thatconnection is made between parts of the production bore, and parts ofthe two auxiliary bores to provide effective connection for these bores.5. A xmas tree as claimed in claim 4 wherein the connections in theproduction and annulus bore are provided by hollow pin and socketconnections.
 6. A xmas tree as claimed in claim 2 wherein if no powercable is used one auxiliary bore is blocked and the other auxiliary boreis provided with a valve for annulus control.
 7. A xmas tree as claimedin claim 1 wherein said at least two axially spaced valves in theproduction bore are ball valves.
 8. A xmas tree as claimed in claim 1wherein said valves are flapper valves, disc valves or plug valves.
 9. Axmas tree as provided in claim 1 wherein separate locking means areprovided between the xmas tree housing and the valve block assemblywhich are actuated when the tree cap fits on to the valve block to lockthe valve block to the xmas tree housing, and to provide fine alignmentbetween the tree cap and the housing.
 10. A xmas tree as claimed inclaim 9 wherein the locking means are radially acting dogs which areactuated by axial movement of a ring within the locking means.
 11. Axmas tree as claimed in claim 1 wherein an external surface of the upperend of an valve block is provided with at least one shaped keyway toensure correct angular alignment between the xmas tree and the tree capto facilitate correct engagement of various connections at the topsurface of the tree cap.
 12. A xmas tree as claimed in claim 1 whereinthe xmas tree housing is coupled to a guide frame which is adapted toguide and receive the tree cap when installed to facilitate couplingbetween the tree cap and the tree housing and valve block.
 13. A xmastree as claimed in claim 12 wherein there is provided a permanent guidebase, attached to the wellhead system, including a side guide frame withguidepost spacing of the same dimensions as the guide frame coupled tothe wellhead so that when the tree cap is removed for workover mode, thetree cap can be moved to one side and parked adjacent to the wellhead.14. A xmas tree structure comprising: a xmas tree housing coupled to asub-sea wellhead; a xmas tree valve block located in said xmas treehousing, said xmas tree valve block having a production bore with atleast two valves disposed in series and at least one auxiliary bore forfacilitating annulus access control, said at least one auxiliary borehaving a valve therein, a xmas tree cap coupled to the xmas tree valveblock and to said xmas tree housing, locking means disposed in said xmastree to fasten the xmas tree housing, xmas tree valve block and saidxmas tree cap together, said respective xmas tree valves being remotelyoperable to be selectively moved between an open and a closed positionto control flow of liquid through said xmas tree structure.
 15. A xmastree structure as claimed in claim 14 wherein there are two auxiliarybores; one being an annulus access bore with at least one valve thereinand the other auxiliary bore for receiving a power cable for supplyingpower to a downhole electrical submersible pump (ESP).
 16. A method ofinstalling the xmas tree on a wellhead for use in a variety of welltests, the tree housing having a blow-out preventer stack initiallycoupled thereto in place of a tree cap, said method comprising the stepsor: a) providing a xmas tree housing coupled to the wellhead, the xmastree housing having a top part adapted to be coupled to the blow-outpreventer connector, b) running a xmas tree valve assembly through saidblow out preventer stack for engaging with the xmas tree housing andbeing operable for providing a well barrier to the flow of well fluid,c) closing the well barrier to provide well isolation once the xmas treevalve assembly is located in said xmas tree housing, d) removing theblow-out preventer stack from the xmas tree housing, e) running in axmas tree cap and securing the xmas tree cap to the xmas tree housingand xmas tree valve assembly to provide coupling from production andannulus bores to the surface and from auxiliary ports and bore to thesurface, the arrangement being such that the valves in the productionand annulus bores can be controlled to provide control of well fluid andxmas tree functionality.
 17. A method as claimed in claim 16 wherein themethod includes the step of using the xmas tree in a workover mode, saidmethod including the additional steps of removing the xmas tree cap andparking the xmas tree cap on a further guide frame adjacent to thewellhead and xmas tree housing, and coupling the blow-out preventerstack on top of said xmas tree housing with said xmas tree assemblycontained therein.